It has been found that the optical excitation of an intersubband transition of an in GaAs/AlAsSb semiconductor quantum well by TM polarization light causes an occurrence of a phase modulation effect with response speed of a few picoseconds with respect to TE polarization light without absorption loss (See non-patent document 1). The present inventors have reported an ultrahigh speed optical gate switch to which the phase modulation effect is applied (See non-patent document 2). The optical gate switch has a Mach-Zehnder interferometer shape of a spatial optical system type which is configured by combining optical components with a size of a few mm to 1 cm degree, such as a mirror and a polarization splitter. The introduction of an optical waveguide including the quantum well having the phase modulation effect into one of the optical path of the interferometer allows it to function as the optical gate switch. By using the optical gate switch, reports have been made of an operation such as a de-multiplexing of an optical time-division multiplexing signal of 160 Gb/s towards a signal of 40 Gb/s, or wavelength conversion of the signal of 160 Gb/s, and a fundamental demonstration towards a device application of an ultrahigh speed phase modulation effect has been made.
An unstable operation of the gate switch becomes a problem because the optical gate switch of the spatial optical system type has the interferometer with a large size. In terms of practicality, it is desirable that the optical gate switch be minimized by producing an optical integrated circuit using components which configure the optical gate switch by using a semiconductor process technology.
In designing of the optical integrated circuit for the optical gate switch using the phase modulation effect caused by the intersubband transition, it is necessary to note the following. When TM polarization control light is propagated through the optical waveguide, it is absorbed by the intersubband transition. Thus, a refractive index of the quantum well is changed and a phase modulation is generated with respect to TE polarization signal light. On the other hand, in the optical waveguide for the phase modulation, the TE polarization is not optically attenuated by the intersubband and interband light transitions in the quantum well. For this reason, efforts are necessary in order to guide TM control light to a portion that requires the phase modulation within the optical circuit.
From the above-described viewpoint, a wavelength converter of the Michelson interferometer type using the phase modulation effect in a semiconductor optical amplifier (SOA) is disclosed as a device having a shape similar to the present invention (See non-patent documents 3 and 4). It is similar to the present invention in that the control light which is emitted from fiber is directly guided to a phase modulation unit of the optical waveguide, and that one of reflection side arms of the Michelson interferometer becomes the phase modulation unit.
While non-patent document 3 discloses that the Michelson interferometer itself is configured by an optical fiber or the coupler, non-patent document 4 discloses that the Michelson interferometer is monolithically integrated on a substrate with the semiconductor optical amplifier (SOA) effect. These documents use the phase modulation effect caused by an optical non-linearity of the semiconductor optical amplifier (SOA), and do not cover details of a technology which is necessary when the phase modulation effect caused by the intersubband transition is optically integrated. In addition, a mechanism which maintains a balance of the light intensity between optical paths of the interferometer is different from the present invention.